The present invention relates generally to fuel cells, and, more particularly, to increasing the rate of hydrogen generation to power such fuel cells.
The fuel cell technology relating to hydrogen-proton exchange membrane (H2-PEM) is of increasing interest as an alternative source of energy. A major concern is the use of pressurized H2 and O2/air containers. H2 is difficult to liquefy, yet the means of generating it only as needed is essential for the efficient working of PEM cells.
While hydrogen gas itself is probably the most efficient source in terms of power generation, there are concerns regarding the handling of hydrogen gas. While many of these concerns may not be wholly justified, nevertheless, alternative fuel sources must be found that will allay most, if not all, of such concerns.
In the PEM cells, the cathode is typically Pt and the anode is also Pt. The proton exchange membrane is located between the two electrodes. On the surface of the Pt anode, one mole of hydrogen generates two moles of proton ions (H+) and 2 moles of electrons.
One promising fuel source of hydrogen is sodium borohydride (NaBH4); see, e.g., S. Amendola et al, xe2x80x9cSystem for Hydrogen Generationxe2x80x9d, PCT WO 01/51410, Jul. 19, 2001. This reference discloses the generation of hydrogen for, e.g., fuel cell applications, using NaBH4. This compound offers the best source of hydrogen, except for hydrogen gas, over other compounds under consideration, such as methanol. Methanol is used in the so-called direct methanol fuel cell (DMFC), a variant of the hydrogen PEM fuel cell. Typically, aqueous methanol, approximately 3 wt %, is used as the fuel. From an energy standpoint, the DMFC provides less power than the hydrogen PEM fuel cell.
There are two outstanding issues with the NaBH4 solution: stability and reactivity. Since NaBH4 hydrolyzes slowly in water, the solution stability is obtained by reducing the reaction rate in an alkaline condition. Several commercially available NaBH4 solutions containing a high concentration of sodium hydroxide have demonstrated excellent stability in long-term storage. These solutions, however, are not sufficiently reactive to carry out the reaction at room temperature as described in Eqn. 1, even in the presence of a metal catalyst: 
Therefore, these stable solutions are not suitable for fuel cell applications, where the operating temperature is near ambient temperature. By lowering the strong base concentration, i.e., to 10 wt %, the solutions are very reactive in the presence of a catalyst at ambient temperature, but these solutions lack the stability in long term storage. Over time, both the gradual loss of sodium borohydride and pressure build-up due to H2 present problems in shelf life and packaging. Clearly, there is a delicate balance between solution stability and reactivity from a formulation perspective using the borohydride solution.
Thus, what is needed is a borohydride solution that evidences long term stability and high reactivity.
In accordance with the embodiments disclosed herein, a proton-exchange-membrane fuel cell is provided with a source of hydrogen gas from a reaction chamber, wherein a mechanism is provided for introducing first and second solutions into the reaction chamber to decompose NaBH4 in the presence of a catalyst to generate hydrogen. The first solution comprises (a) 5 to 50 wt % MBH4, where M is an alkali metal, (b) 5 to 40 wt % alkali hydroxide or alkaline metal hydroxide, and (c) the balance water, and the second solution comprises (a) 51 to 100% water, and (b) the balance, if any, comprising at least one water-soluble component.
Also in accordance with additional embodiments disclosed herein, a method of operating the proton-exchange-member fuel cell is provided. The method comprises introducing the first and second solutions into the reaction chamber to form the mixture.
The approach taken herein of mixing two aqueous solutions in the presence of a catalyst solves the stability and reactivity issues of aqueous alkali borohydride solution simultaneously. Further, the approach taken herein of reactive mixing enables one to optimize for the variables in two opposing considerationsxe2x80x94stability versus reactivity. The alkali borohydride solution can be formulated to provide high specific energy as well as excellent long term stability. Upon mixing with a second aqueous solution, the borohydride solution becomes xe2x80x9creactivexe2x80x9d and provides an accelerated reaction rate and high percent conversion toward hydrogen evolution.